1. Field
The present invention relates to a fire-resistant safety cable. More particularly, the present invention relates to a substantially flat fire-resistant cable, which comprises at least two electrical conductors that are adjacent to one another.
2. Description of Related Art
Safety cables are especially power-transporting or data-transmitting cables, such as for control or signaling applications.
Fire-resistant safety cables must, in a fire, maintain an electrical function. Preferably, said cables must also not propagate the fire. Said safety cables are used for example for lighting emergency exits and in elevator installations.
Fire-resistant cables must meet the criteria, for example set by the French standard NF C 32-070. According to this standard, the cable is placed horizontally in a tube furnace, the temperature of which is raised to 920° C. and held there for 50 minutes. The cable must not undergo a short circuit during this temperature rise and during 15 minutes at 920° C. Throughout this time, to simulate the falling of objects in a fire, the cable is periodically subjected to a shock by a metal bar in order to shake the cable.
Cables passing the test defined by NF C 32-070, paragraph 2-3 belong to the CR1 category.
Criteria similar to those defined in French standard NF C 32-070 are also defined by international standards, such as IEC 60331, or European standards, such as EN 50200.
Documents JP 01-117204 and JP 01-030106 disclose two fire-resistant flat cables, said cables comprising several conductors surrounded by an insulator and by a polyethylene outer jacket, the insulating layer of each electrical conductor consisting of mica tapes.
The Applicant has noticed that a fire-resistant cable provided with an insulating layer consisting of mica tapes has several drawbacks. In particular, such a cable may have a gap (or space exposing the conductor) in the mica tape wrapping, thereby causing a fault in the protection of the conductors, leading to a short circuit.
Fire-resistant cables having an approximately round cross section are also known.
For example, document EP 942 439 discloses a fire-resistant halogen-free round safety cable, comprising at least one conductor, an insulator around each conductor, and an outer jacket, empty spaces being provided between said jacket and said insulator of each electrical conductor.
The insulator of each conductor is made of a composition formed from a polymeric material containing at least one ceramic-forming filler capable of being converted, at least on the surface, to the ceramic state at high temperatures corresponding to fire conditions.
The outer jacket is made of a polyolefin composition containing at least one metal hydroxide filler.
The Applicant has noticed that a fire-resistant cable having a round cross section has several drawbacks. For example, in a fire, a fire-resistant cable having a round section has a high risk of contaminating the insulating layer with the ash resulting from the combustion of the outer jacket. The Applicant has noted that this is especially due to the reciprocal arrangement of the insulated elements. This is because, in the case of a cable comprising more than two insulated elements, at least one insulating element is superposed on the others so as to provide the cable with a round cross section. An insulated element generally comprises an electrical conductor and an insulating layer surrounding said conductor.
In the case of a fire-resistant cable having a round cross section, the outer jacket is generally converted, through the action of a fire, to ash, which may impede the conversion of the polymeric material of the insulator to a ceramic, causing the appearance of cracks in the insulator of the conductor.
Furthermore, the superposition of the insulated elements may cause the size of the cracks to increase appreciably, resulting in collapse of the insulating layer(s) contaminated by said ash. These drawbacks result in a reduction in the insulating protection provided by the insulating layer(s) of the cable and to an increase in the risk of short-circuiting the conductors. These risks relate in particular to the superposed insulated elements.
Furthermore, this ash may cause the volume and surface conductivity of the insulation to increase, which would impair the proper operation of the cable.
In addition, the insulated electrical conductors (or insulated elements) used in round fire-resistant safety cables are generally twisted.
The twisting of the insulated elements leads to the existence of multiple contact zones between said insulated elements, especially when based on three elements, incurring risks of short-circuiting, for example when the insulator has defects in its structure, such as cracks that may be created during conversion of the insulator on the conductors to ceramic at high temperature.
Moreover, in a fire, objects such as a beam or elements of a building structure may fall and strike the cable, and thus damage the latter or impair the mechanical integrity of the insulator converted to ceramic, or in the process of being converted to ceramic, of each element. The fall of such an object may cause, in the case of twisted elements, an insulated element to be compressed between said object and another element of the same cable, damaging the insulator converted to ceramic or in the process of being converted to ceramic, and thus short-circuiting the two conductors.
Furthermore, the twisting of the cable elements generally results in the formation of mechanical stresses that remain within the Cable and are released during a fire, which may damage the insulation material of the cable during its conversion to a ceramic layer.
There is therefore a need for a fire-resistant cable that allows the abovementioned drawbacks to be alleviated.